Tutorial on Wall Borders?
A typical way would be simply to duplicate the bottom edge of the wall, and convert it to a curve, down which you can sweep a profile of the molding.


ShiftD Duplicate the bottom edge of the wall
P separate it into its own object, and Header > Object Menu > Convert it to a curve. (The 'Path'). Make the path a 2D curve, to ensure clean miters.
Construct a profile for the molding, either as a mesh which you convert to a curve, or as a curve in the first place. (The 'Profile')




Note the orientation of the profile curve with respect to its object axes, and the position of its origin at its bottom left corner (shown with the 3D Cursor). All this can be adjusted in Edit Mode.


Now just assign the 'Profile' curve as the Bevel Object of the 'Path' curve.




You might want to convert the 'Path' curve back to a mesh when you're done, to make Auto Smooth available for it. That way, you can keep the poly count down.